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DFO  GC-1205A

The monthly mean water levels in the St. Lawrence were exceptionally low last spring. Such low water levels are generally recorded at the end of the summer, making the 2010 season exceptional.

In the Port of Montréal, the Canadian Hydrographic Service recorded mean water levels of 0,60 m in April, -0,15 m in May and -0,22 m in June with reference to chart datum. The level for the month of May was the lowest ever recorded for that month since records were first kept (1913-2010). As for June, the only year lower levels were recorded was in 1965.

The impact of low water levels is felt primarily in the section of the St. Lawrence River between Trois-Rivières and Montréal as well as in Lac Saint-Louis. The tide has no influence in this portion of the river. Farther downstream, the tide compensates for low water levels during part of the day, and offers shipping a margin for manoeuvre.

This situation is the result of mild temperatures and low snow accumulation last winter. These conditions reduced the spring freshet, which had a direct impact on water levels in streams. However, since water level is closely tied to weather conditions, consistent rainfall over the Great Lakes and Ottawa River watersheds could change the forecast and restore the situation in the St. Lawrence to normal.

Monthly mean water levels below chart datum affect both recreational boaters and mariners, and can pose hazards to the safety of people and the environment. For commercial mariners, low water levels also mean that they must reduce their cargo weight. As for recreational boaters, they must make sure they stay in the deepest channels.

The Canadian Hydrographic Service is responsible for measuring water depth, monitoring and predicting water levels, establishing chart datum and mapping waterways.

Tips for safe boating

Safe boating begins with proper preparation. Before setting out, you should:

  • Obtain information on water levels from the Internet or by dialing the toll-free number 1-877-775-0790;
  • Keep the most recent nautical charts on board and use them;
  • Update your nautical charts using the notices to shipping and the notices to mariners;
  • Check water levels in the area where you are navigating to make daily corrections to the depths indicated on the charts. The water level value must be added to or subtracted from (when it is negative) the depth indicated on the chart;
  • Take into account that low water levels affect the position of buoys. In such conditions, buoys may not be sufficient for accurate identification of a navigation zone or a channel that is safe for certain vessels;
  • Do not venture into unfamiliar areas without obtaining information about obstacles and obstructions;
  • Reduce your vessel’s speed where the current permits; and
  • Know the draft of your vessel.

Denis Lefaivre